In 1998, the Chief Medical Officer’s Expert Advisory Group on Chlamydia trachomatis considered the evidence-base associated with screening for genital chlamydial infection. This group concluded that chlamydia screening met the criteria for a screening programme and recommended that one be established⁽¹⁾.

Here we provide an overview of the current evidence-base for chlamydia screening.

Following the Chief Medical Officer’s recommendation, chlamydia screening pilot studies^(2,3) were conducted in two areas of England. Screening was offered opportunistically to young women attending general practice, family planning services, genitourinary medicine (GUM) clinics, adolescent sexual health clinics, termination of pregnancy clinics, and women’s services in hospitals. These pilots demonstrated that opportunistic screening using urine samples is practical and acceptable, and that the frequency of chlamydia infection in women using these services (including non-GUM services) was substantial.

In addition, the Chlamydia Screening Studies (ClaSS) project⁽⁴⁾ included a cross-sectional survey of 19,773 women and men aged 16-39 years selected from general practice lists. Participants were invited to collect urine and (for women) vulvo-vaginal swab specimens at home and post them to a laboratory for chlamydia testing. This study demonstrated that the frequency of chlamydia infection in the general population was highest in those below 25 years of age, and was similar in men and women. Self-collected urine specimens and vulva-vaginal swab were found to be suitable samples for diagnostic testing with Nucleic Acid Amplification Tests (NAATs).

In relation to the effectiveness of chlamydia screening, randomised controlled trials have demonstrated reductions in the risk of pelvic inflammatory disease (PID) among women screened for chlamydia:

• Scholes et al 1996⁽⁵⁾ conducted a randomized controlled trial in the United States (US) to determine whether screening for chlamydia reduced the risk of PID. 1,009 women were randomly assigned to screening and 1,598 to usual care. At 12 months the risk of PID was 18/10,000 woman-months in the controls and 8/10,000 woman-months in the screened group, giving a relative risk reduction of 56% among those screened.
• Ostergaard et al 2000⁽⁶⁾ conducted a cluster-randomized trial in Denmark to determine whether the offer of home-sample screening for chlamydia can reduce prevalence of infection and diagnoses of PID in women. 2,603 women were assigned to home-screening and 2,884 offered clinic services. At 12 months the prevalence of chlamydia was 2.9% in the intervention group and 6.6% in the control group. 2.1% of the intervention group had been treated for PID compared to 4.2% in the control group, giving a relative risk reduction for PID of 50% for those in the intervention group.
• Oakeshott et al 2010⁽⁷⁾ conducted a community-based trial among sexually active female students in London. 2,529 women provided self-collected vaginal swabs which were then randomised to either immediate testing and treatment or storage for 12 months prior to testing. At 12 months, the incidence of PID was 1.3 in the screened group and 1.9% in the control group, giving a non-significant relative risk reduction 35% for those in the intervention group. Most episodes of PID occurred in women who tested negative for chlamydia at baseline (79%). 9.5% of controls who tested positive for chlamydia at baseline developed PID over the following 12 months.

Observational data from Sweden and the US showed reductions in chlamydia during the 1990s following increased opportunistic screening^(8-10). However, more recent data suggests that some of these reductions have not been sustained⁽¹¹⁾.

Declines in the incidence of pelvic inflammatory disease and ectopic pregnancy have also been reported from a number of countries conducting chlamydia screening including Sweden, the US, and Canada^(10,12-15).

Mathematical modelling has indicated that annual screening of 26% of men and women <25 years of age with 20% partner notification would reduce the population prevalence of chlamydia by 29% after 1 year, 68% after 5 years and 82% after 10 years. If screening coverage was increased to 43%, the estimated reductions in prevalence were 40% after 1 year, 79% after 5 years and 89% after 10 years. Increasing the rates of partner notification resulted in even greater declines in prevalence⁽¹⁶⁾.

Other mathematical modelling studies have predicted more moderate reductions in prevalence following the introduction of chlamydia screening, due to differences in the assumptions used ⁽¹⁷⁾. There is considerable uncertainty, and scientific debate, about several important parameters used in the mathematical models of screening effectiveness including: the prevalence of undiagnosed/untreated chlamydia, patterns of sexual contacts and chlamydia transmission, and the rate of progression to PID and other sequelae. In particular, since the NCSP began in 2003, there have been a number of studies reporting lower rates of progression from chlamydia infection to PID and other sequeale than previously reported. The strength of these studies has, however, been debated, and the difficulties/limitations of research in this area highlighted ^(18-26).

Finally, some recent reviews of the evidence for chlamydia screening include:

• U.S. Preventive Services Task Force. Screening for chlamydial infection: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2007 Jul 17;147⁽²⁾:128-34.
• Low N, Bender N, Nartey L, Shang A, Stephenson JM. Effectiveness of chlamydia screening: systematic review. Int J Epidemiol. 2009 Apr;38⁽²⁾:435-48.
• Scottish Intercollegiate Guidelines Network. Management of genital Chlamydia trachomatis infection. A National Clinical Guideline. March 2009 www.sign.ac.uk/pdf/sign109.pdf